1. Field of the Invention
This invention relates generally to methods for seismic prospecting and more specifically to a method which optimizes the use of the energy available from a seismic source.
2. Description of the Prior Art
In the practice of seismic prospecting, acoustic energy is introduced into the earth at or near its surface by one of a variety of acoustic sources. Typical sources in a land prospecting environment include dynamite and vibrators. In marine prospecting it is customary to use air guns. In either case, an array of seismic receivers is arranged in the vicinity of the source. In the case of land prospecting this typically comprises a plurality of geophone groups arranged along a line extending from the source point, or in some cases in an areal distribution near the source point. In the case of marine prospecting, the seismic receivers typically comprise a plurality of hydrophones towed behind the boat in a seismic streamer.
When the source is fired, the energy propagates down through the material of the earth in the form of elastic waves. These waves are reflected by impedance discontinuities that occur at the boundary between layers of different subsurface materials. The reflected energy propagates back upward and is detected by the seismic receivers. Examination of the received signals by skilled analysts permits judgments to be made about the subsurface structure.
Typically, for each firing of the source, the data from the seismic receivers is recorded for a sufficient period of time T.sub.max to ensure that all reflections from the deepest reflector of interest are included in the record. Recording is then discontinued until the next source excitation is to occur. As a result there is no intermingling of reflections from two or more shots on any given record. Typically the sources are fired at intervals of ten seconds or more.
In most cases, processing and interpretation of the signals received by the receivers is performed in the time domain. It is also possible, however, to transform the signals and perform the processing in the frequency domain. Typically this would involve digitizing the received signals and transforming them by means of the discrete fourier transform (DFT). The output of the DFT is in the form of transformed values at each of a plurality of discrete frequencies. In this case, the frequency interval .DELTA.f between the plurality of discrete frequencies at which independent data exists is determined by the recording time T.sub.max. Specifically EQU .DELTA.f=1/T.sub.max